The invention generally relates to stall protection circuits for motors and, in particular, a stall protection circuit for an electronically commutated motor wherein the circuit is part of the drive control for the electronically commutated motor.
Some electronically commutated applications experience excessive temperature rise at even normal operating current levels after operation under stall conditions. This is due to the inability of the motor to dissipate heat built up during the stall condition. In a stall condition, the lack of air circulation inhibits heat dissipation. Hermetically sealed motors are particularly sensitive to temperature rises at or after stall conditions. Some hermetically sealed motors under normal operation take full advantage of the cooling capability of refrigerants being pumped through a system with which the motor is associated. These motors are particularly sensitive to temperature rise at stall when refrigerants are not being pumped and the cooling capability has been lost.
The requirement for stall protection has typically been met by monitoring the temperature and current of the motor. In particular, temperature sensors are physically attached to the stator windings and/or current sensitive sensors are placed electrically in series with the windings. In certain electronically commutated motor applications, the use of such sensors is complicated by two factors. First of all, as the electronically commutated motor drive provides a three-phase current source to the motor, the current sensors have to be three-phase protectors. Second of all, as the electronically commutated motor drive regulates the current, the temperature sensors must operate solely on winding temperature rise, which slows their operation. In addition, this slower operation causes a longer protection cycle requiring a significantly longer test period to demonstrate the multicycle capability of the device in a particular application.
In many electronically commutated motor systems the motor drives a compressor drive and provides a speed reference signal to a system controller. In these systems, the combination of the electronically commutated motor and the system controller can be configured as a protection circuit for stall conditions. However, the extra requirements placed on the system controller can be very burdensome. It would be desirable to provide a stall protection circuit which minimizes the above disadvantages and provides responsive protection to stalling.